Pressure reducing device

Abstract

The present invention relates to a pressure reducing device and to an apparatus for detecting aerosols and particularly aerosols of solid particles emitting α rays. This apparatus comprises a measuring chamber equipped with a sampling filter and a radiation detector and linked with the outside by an inlet pipe and an outlet pipe. The inlet pipe is equipped with a venturi tube making it possible to produce a pressure reduction in the measuring chamber and exert on the solid particles of the aerosol fluid flow forces exceeding the forces tending to make these particles adhere to the walls. Application to the detection of α radiation emitting aerosols.

What is claimed is: 
     
       1. An apparatus for detecting an aerosol for α-ray emitting solid particles, comprising: means defining a measuring chamber;   a sampling filter in said measuring chamber;   a radiation detector in said measuring chamber;   outlet means communicating with said measuring chamber;   an aerosol source;   inlet means communicating said aerosol source with said measuring chamber;   a venturi tube in said inlet means; and   means for causing said aerosol to flow through said venturi tube at or near the speed of sound, whereby a fluid flow force of said aerosol reduces adherence of particles of said aerosol on walls of said venturi tube.   
     
     
       2. An apparatus according to claim 1, wherein the venturi tube includes a convergent portion having an apex angle below 30°, a central cylindrical portion, whose length is equal to a diameter thereof and a divergent portion having an apex angle below 10° C. 
     
     
       3. An apparatus according to claim 2, wherein the apex angle of the convergent portion of the venturi tube is 21° and the apex angle of the divergent portion 7°. 
     
     
       4. An apparatus according to claim 1, wherein the inner walls of the venturi tube are made from an electrically conducting material. 
     
     
       5. An apparatus according to claim 1, wherein the neck and the divergent portion of the venturi tube are unitary, without any connection. 
     
     
       6. An apparatus according to claim 1, wherein the inner walls of the venturi tube are covered by a surfactant. 
     
     
       7. An apparatus according to claim 6, wherein the surfactant is a fluorine-containing product.